I'm testing running HAProxy as a dedicated load balancer behind Apache 2.2, replacing our current configuration where we use Apache's load balancer. In our current, Apache only, set-up if all the backend (origin) servers are down Apache will serve a 503 service unavailable message. With HAProxy I get a 502 bad gateway response.

I'm using a simple reverse proxy rewrite rule in Apache

RewriteRule ^/(.*) http://127.0.0.1:8000/$1 [last,proxy]

In HAProxy I have the following (running in default tcp mode)

defaults
    log             global
    option          tcp-smart-accept
    timeout connect 7s
    timeout client  60s
    timeout queue   120s
    timeout server  60s

listen my_server 127.0.0.1:8000
    balance leastconn
    server backend1 127.0.0.1:8001 check observe layer4 maxconn 2
    server backend1 127.0.0.1:8001 check observe layer4 maxconn 2

Testing connecting directly to the load balancer when the backend servers are down:

[root@dev ~]# wget http://127.0.0.1:8000/ test.html
--2012-05-28 11:45:28--  http://127.0.0.1:8000/
Connecting to 127.0.0.1:8000... connected.
HTTP request sent, awaiting response... No data received.

So presumably this is down to the fact that HAProxy accepts the connection and then closes it.